(1) Field of the Invention
The present invention relates to phase synchronization, particularly of digitally synthesized analog signals, and especially to the phase synchronization of signals commensurate with audio tones generated in an electronic musical instrument. More specifically, this invention is directed to circuitry for synchronizing the phase of plural signals of the same frequency when in digital form. Accordingly, the general objects of the present invention are to provide novel and improved methods and apparatus of such character.
(2) Description of the Prior Art
While not limited thereto in its utility, the present invention is particularly well suited for use in electronic musical instruments. Such instruments, keyboard instruments of the electronic organ type for example, were initially analog devices. It is, of course, comparatively easy to synchronize the phase of simultaneously generated analog signals having the same basic frequency. This may, for example, be accomplished through the use of phase regulating loops and such phase regulation circuitry was common in early electronic musical instruments. In many more recent electronic musical instruments, however, the notes are synthesized digitally and each note which can be simultaneously played will have an associated "phase counter". Since the moment sound production commences, i.e., the moment when the musician touches a key, cannot be predicted, there are unforeseeable phase relationships between the individual notes which may be simultaneously produced. As long as the notes are of different basic frequency, these unforeseeable phase relationships are of no great significance to the audio frequency tone which is produced. However, if simultaneously sounded notes have the same basic frequency or the identical frequency, and this may be the case if the same note is simultaneously "struck" on two parallel manuals (keyboards), then interferences occur and these interferences are different depending on the relative times the simultaneously depressed keys are operated.
The problem discussed above could be solved simply by associating with each manual, or with each octave of an instrument, its own complete synthesizing circuit whereby superimposition would take place only after the conventional digital-to-analog converter, i.e., immediately prior to delivery of the analog signal to the sound producer. This solution, however, requires a rather substantial duplication of circuitry and thus adds to the complexity and cost of the instrument.